Well, it isn't easy because most methods used to determine your best distance are far from foolproof. For instance, competitive potential is, to some extent, determined by muscle composition. If your sinews are mainly composed of fast-twitch fibers, for example, you'll tend to excel in shorter events like the 800 and mile. On the other hand, if your muscles are mostly slow-twitch, you may have trouble generating explosive blasts of power. You'll probably find your home in longer races like the 10-K or marathon.

But here's the catch: some fast-twitch runners have won major marathons, and some slow-twitch competitors have been excellent milers.

Then there's body type. It's true that the best short-distance folks tend to be tall and brawny, since such traits increase stride length and maximize muscle power. Being tall and muscular can also be a disadvantage in longer events because it's fatiguing to lug hefty bones and muscles over long distances. That's why great marathoners tend to be rather small-framed figures.

But again, body type, like muscle composition, is not a true-blue performance predictor. Some tall runners have been great marathoners, and some short folks have been outstanding milers or sprinters.

Perhaps a better race predictor is Horwill's Law (named after British coach Frank Horwill), which says that as race distance doubles, running speed should slow down by no more than an average of 16 seconds per mile. If you run your 5-Ks at 7 minutes per mile, but your 10-Ks lag along at 7:25 pace (instead of 7:16), your half-marathons at 7:40 (instead of roughly 7:32) and your marathons a bit slower than 8:00 (instead of 7:48 or so), it's tempting to say the 5-K is your best event.

Tempting, until you consider that a marathon usually looks the worst under the harsh glare of Horwill's Law. Relatively slow marathon times are not ordinarily caused by lack of ability. They're usually related to problems that don't occur in shorter races (carbohydrate depletion, dehydration, debilitating blisters and unwise use of sports drinks and gels).

In the final analysis, performances from the 5-K to the marathon tend to depend far more on motivation and training than on ability. If you love the marathon but detest the 5-K, you'll do better in the longer event. And if you prefer short, speedy intervals on the track but seldom do the work required for the 10-K and marathon, you'll perform best in the 5-K.

To put this all together, take your body type, personal preferences and Horwill's Law into account. If you're a tall, muscular runner who loves speed, and you have mile splits that slow significantly as you add distance, your perfect race might be the 5-K. On the other hand, if you're a short, wiry runner who loves to go long, then the marathon may be your game. But don't rule out anything. With the right training and motivation, you should be able to perform equally as well at distances ranging from the 5-K to the marathon. Here's an action plan for making any distance your perfect one. (Don't do any of these workouts more than once or twice a week.)

* 5-K: Run 1200s or 1600s at your current 5-K race pace. Do three to four per workout, with 3- or 4-minute recoveries.

* 10-K: Do some longer intervals (2000s and 2400s) at your 10-K pace. Do three per workout with 5-minute recoveries.

* Half-marathon: Run 5 miles at a pace that's 15 seconds per mile slower than your 10-K race pace.

* Marathon: Do some 12- to 13-mile runs at your goal marathon pace (approximately 30 to 40 seconds slower per mile than your 10-K race pace) and some shorter runs at lactate threshold pace, which is slightly slower than 10-K race pace. For your long 18- to 20-mile runs, warm up for a couple miles, amble along for the next 12, hit 3 to 4 miles at your marathon goal pace and then cool down.

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